Work out how hot it actually feels when humidity is added to the air temperature.
Heat exhaustion likely; heatstroke likely with continued activity.
How humidity makes heat dangerous
Your body cools itself by evaporating sweat. The more moisture the air already holds, the slower that evaporation runs, so the same air temperature feels far hotter and your body struggles to shed heat. That is why 32 °C at 20% humidity is uncomfortable while 32 °C at 80% humidity can be genuinely dangerous.
In high heat, drink before you feel thirsty, move activity to early morning or evening, wear loose light clothing, and take breaks in the shade. Children, older people and anyone on medication that affects sweating are at higher risk at the same heat index.
At 32 °C, humidity alone moves the apparent temperature by 19 degrees
Your body cools by evaporating sweat, and humid air accepts very little more water. So the same air temperature is survivable in dry heat and dangerous in wet heat. At 32 °C the apparent temperature runs from 30 °C at 20% humidity to 49 °C at 90% — a nineteen-degree spread from a number most forecasts mention in passing.
How it works
- Applies the Rothfusz regression used by the US National Weather Service to combine air temperature and relative humidity.
- Reports the apparent temperature — what the heat does to a body, rather than what the thermometer reads.
- Places the result against the recognised caution, extreme caution, danger and extreme danger bands.
the Rothfusz regression is a nine-term polynomial in temperature and relative humidity, fitted to human heat-balance modelling rather than derived from first principles it is valid above roughly 27 °C; below that the correction is negligible adjustments apply at very low humidity and at very high humidity with moderate heat
Worked example
Holding the air temperature at 32 °C and varying only the humidity.
- 20% humidity → feels like 30.0 °C, slightly cooler than the air
- 40% → 32.3 °C, essentially the air temperature
- 60% → 37.1 °C
- 70% → 40.4 °C
- 90% → 49.0 °C
Nineteen degrees of apparent temperature separate a dry 32 °C day from a humid one. For comparison, 40 °C at 25% humidity feels like 41.1 °C — almost identical to 32 °C at 70%.
Reading the result
- Below about 40% humidity the apparent temperature can fall below the air temperature, because evaporation is efficient enough to cool you faster than the air heats you. That is why dry heat is genuinely more tolerable and not merely a saying.
- The heat index assumes shade and light wind. In direct sun the effective figure can be roughly 8 °C higher, which is not a small correction when you are already in the danger band.
- It is modelled on a notional adult. Age, medication, acclimatisation, exertion and clothing all shift the real risk, and none of them appear in the formula.
- Sustained exposure above about 40 °C apparent temperature carries real risk of heat exhaustion; above 54 °C, heatstroke becomes likely. These are thresholds for planning outdoor work, not merely comfort advice.
Common questions
- Why does humidity matter so much?
- Because sweating only cools you when the sweat evaporates, and saturated air can absorb very little additional moisture. At high humidity your main cooling mechanism largely stops working, while heat continues to arrive.
- Is the heat index the same as 'feels like'?
- Usually, in warm weather. Most forecasts show heat index above about 27 °C and wind chill below about 10 °C, labelling both 'feels like'. They are different formulas addressing opposite problems.